Common-Drain Amplifier
Also called a source follower for reasons that will become clear shortly
For DC bias, neglect small signal source (and its resistance) and the small-signal load resistance; iSUP = ISUP Note that V OUT = V BIAS V GS The DC gate-source voltage is:
EE 105 Spring 2000
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Week 11, Lecture 24
DC Transfer Curve
p-well CMOS process means that the source and bulk can be shorted ... not true for an n-well process.
The threshold voltage VTn is not a constant, since the source-bulk voltage VSB increases as VOUT increases: V Tn = V TOn + n ( V OUT V ) 2 p 2 p
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EE 105 Spring 2000
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Week 11, Lecture 24
DC Transfer Curve for Common-Drain Amps
VOUT
VBIAS
Simple idea: slope of transfer curve is the voltage gain ... about 1 The common-drain is a voltage buffer
dv OUT A v = ---------------- 1 dv BIAS
EE 105 Spring 2000
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Week 11, Lecture 24
Common-Drain Open-Circuit Voltage Gain
For finding Av exactly, remove the source and its resistance and the load resistance ... apply a test voltage and find the output voltage
KCL at source node:
v out ---------------- g m ( v t v out ) ( g mb v out ) = 0 r oc r o 1 v out ---------------- + g m + g mb = g m v t r oc r o
gm gm v out --------------------------------------------------- ---------------------A v = --------- = g m + g mb vt 1 g m + g mb + ---------------- r oc r o
EE 105 Spring 2000
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Week 11, Lecture 24
Output Resistance of Common-Drain Amplifier
*
Leave the source resistance attached while exercising the output with a test voltage
KCL at the source node ... remove ro || roc and put it back in i t + g m ( 0 v t ) + ( g mb v t ) = 0 v t 1 1 R out = ( r o r oc ) --- = ( r o r oc ) ---------------------- = -------------------------------------------------------------g + g [ 1 ( r o r oc ) ] + g m + g mb it m mb Typically, ro || roc >> gm + gmb 1 R out ---------------------g m + g mb
EE 105 Spring 2000
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Week 11, Lecture 24
Common-Drain Small-Signal Model
Input resistance is infinite: open-circuit from gate-source
If source and bulk can be shorted (possible for a MOSFET in a well), then the gate is essentially 1 (since backgate generator has zero vsb controlling it.) Output resistance is ideally zero for a voltage-output amplifier: typical values gm = 500 S gmb = 50 S 1 R out = ---------------------- 2 k g m + g mb The output resistance can be reduced by increasing the transconductance ... (W/L) can be made huge in order to drive Rout toward zero.
EE 105 Spring 2000
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Week 11, Lecture 24
Summary of MOSFET Two-Port Models
EE 105 Spring 2000
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Week 11, Lecture 24
Assessment of MOS Amplifiers
Common-source is the only stage that provides gain Common-gate can buffer a poor current source into a nearly ideal one Common-drain can buffer a poor voltage source into a nearly ideal one
We need more than one stage to approach an ideal amplifier (of any of the 4 types)
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Week 11, Lecture 24
